The present invention relates to a multi-carrier radio transmitter and a method of power control in a multi-carrier radio transmitter. It has particular application to power control in a base station of a cellular radio network.
In a cellular radio network a geographic area is divided into separate cells. Each cell has a base station for communicating with mobile terminals or the like which are within that cell. Each base station has a receiver for receiving signals from the mobile terminals and a transmitter for sending signals to the mobile terminals. The transmitter communicates with the mobile terminals by modulating a carrier wave. In time division multiple access (TDMA) the transmitter transmits a series of TDMA frames where each frame comprises a succession of time slots and each time slots is associated with a separate communication channel. As an example in GSM the number of time slots per frame is eight. A mobile terminal is assigned a particular communication channel and the base station transmits to that mobile terminal in successive frames by sending signal bursts which occupy an assigned time slot.
Each mobile terminal is in a different environment and at a different distance from the base station. The power level of the signal burst from the base station occupying the slots in a TDMA frame may therefore need to be varied from slot to slot (i.e. from mobile terminal to mobile terminal) or from frame to frame (i.e. as the environment of a mobile terminal changes). Each of the signal bursts will be sent with a predetermined transmit power level which will generally differ from slot to slot. In addition, the transmitter is switched off between signal bursts for a predetermined period of time (the guard period) to separate the individual communication channels. Consequently, on the initiation of a burst, transmitted power must be ramped up from a low value to the predetermined transmit level for that communication channel. Furthermore, at the end of the burst the power level must be ramped down from the predetermined transmit level to a low level. According to the GSM standard guard periods have a duration of about 30 xcexcs, time slots are 577 xcexcs, and the time to ramp a signal burst up to its predetermined level or to ramp it down from its predetermined level is approximately 10 xcexcs. The up and down ramping periods are included in the guard period, the remaining portion of the guard period being a constant low power level period.
To increase the number of channels in a cell it is possible to use a number of single carrier narrow band transmitters in a base station where each transmitter is operating with a particular carrier frequency. The allocation of different carrier frequencies to different channels is referred to as frequency division multiple access (FDMA). In each such narrow band, single carrier, transmitter power control is typically achieved by comparing a sample of output power with a reference signal, the output power being adjusted in dependence upon that comparison. U.S. Pat. Nos. 5,334,979, 5,337,006, 5,128,629, 5,603,106, 5,303,268, 5,126,688, 5,182,527 and EP 0369135 describe adjusting output power by varying the single carrier signal at an RF frequency using controllable attenuators or variable gain amplifiers. WO 9302505 and U.S. Pat. No. 5,193,223 perform the adjustment of a single carrier signal at an intermediate frequency. U.S. Pat. No. 5,293,407 describes a digital adjustment of the power level.
A preferred approach for increasing the number of channels in a cell, is to use a multi-carrier broad band transmitter to implement parallel multiple access. FIG. 1 illustrates a transmitter in which first, second and third digital signals 2, 4 and 6 are respectively input to first, second and third modulators 10, 12 and 14 to modulate carriers having frequencies F1, F2 and F3 and produce respective first, second and third digital modulated signals 16, 18 and 20. Each of the first, second and third digital signals 2, 4 and 6 is a stream of data bits be transmitted. Each stream of data bits controls a modulator to produce a digital modulated signal which itself is composed of a stream of digital words.
Each one of the digital modulated signals 16, 18 and 20 is a digital representation of an analogue carrier having respectively frequencies F1, F2 and F3 modulated by respective ones of the first, second and third digital signals 2, 4 and 6. The modulated signals 16, 18 and 20 are input to an adder 22 which combines the signals to create a digital multi-carrier signal 24. The digital multi-carrier signal 24 is input to the first intermediate frequency (IF) block 26, comprising a digital to analogue converter 28, a band pass or low pass filter 30 and an amplifier 32 in series to produce a multi-carrier analogue signal, first IF signal 34. This signal is continuous in time and amplitude in comparison to the digital multi-carrier signal which is discrete in time and amplitude.
This signal is passed to the second IF block 36, comprising a mixer 38, a band pass filter 42 and amplifier 44 in series and a local oscillator 40, to create a second intermediate frequency (IF) signal 45. The second IF signal 45 is supplied to a radio frequency block 46, comprising a mixer 48 and a band pass filter 52 in series and a local oscillator 50. The output of the radio frequency block 46, a radio frequency signal 53, passes in series through a linear power amplifier and a band pass filter 56 to produce a power amplified radio frequency signal 57 which is then transmitted by an antenna 58. As an example, the radio frequency signal has a carrier with a frequency range of 925-960 MHz. The multiple signals are combined in digital format, before conversion to analogue. In a TDMA system the slots and frames of the different carriers are synchronised.
The multi-carrier transmitter therefore operates in parallel and does not have separate transmitter components for each carrier wave, which allows wide band multi-carrier transceivers to be of reduced cost and size.
It is an aim of embodiments the present invention to provide power control in a multi-carrier radio transmitter.
According to one aspect of the present invention there is provided A multi-carrier radio transmitter comprising combination means for receiving and combining a plurality of carriers including a first modulated carrier for transmission in a first channel and a second modulated carrier for transmission in a second channel, to create a multi-carrier signal; and open loop power control means arranged to individually vary the power of each of the plurality of carriers before said combination and closed loop power control means arranged to vary the output power of the transmitter, the closed loop power control means being configured to operate on the multi-carrier signal after the combination means.
According to another aspect of the present invention there is provided a method of power compensation in a multi-carrier radio transmitter, wherein a plurality of carriers are combined to create a multi-carrier signal, said plurality of carriers including a first modulated carrier for transmission in a first channel and a second modulated carrier for transmission in a second channel, the method comprising the steps of:
(a) determining the transmit power level required in each carrier;
(b) varying the power level of each carrier to the determined levels using open loop power control;
(c) combining the carriers to form the multi-carrier signal; and
(d) compensating for changes in the power level of said multi-carrier signal using closed loop power control, by:
detecting said multi-carrier signal; and
adjusting the power of said multi-carrier signal in dependence upon said detection.
Preferably in both aspects, the open loop power control is configured to effect relatively fast power variations and the closed loop power control is configured to effect relatively slow power variations.
Power control is particularly important in a cellular network when TDMA is being implemented. The size of the cell is determined by the maximum transmitted power of the base station within that cell. It is therefore necessary to limit the maximum transmitter power for all transmitters in a cell to control the overlap of neighbouring cells and interference resulting therefrom. This type of power control will henceforth be referred to as static power control. Static power level may be changed in connection with network replanning. Power control is also needed in TDMA to control the ramping of a burst signal at the beginning and end of a transmission burst. This is called power ramping. In addition, the attenuation in a communication channel between a base station and a mobile terminal may change significantly in short periods of time, for example, as a mobile terminal moves behind an obstruction. It is therefore important that in each communication channel, defined by a carrier frequency and a time slot, the power level at which the base station transmits to a mobile terminal can be altered for each time slot. This type of power control will henceforth be referred to as dynamic power control. Dynamic power control does not affect the output power during a burst but between successive bursts. The difference between bursts may be as much as 30 dB. Dynamic power control and power ramming together constitute what will in the following discussion be called fast power control. Finally, due to variations in temperature and ageing the output power of a transmitter can vary over time, and this variation should be compensated, which is henceforth referred to as slow power control. The responsivity required for slow power control is mainly dictated by the effect of temperature variations in the power amplifier. This may require compensation every few seconds or minutes.
Embodiments of the present invention preferably have separate fast and slow power control. The fast control is preferably done in an open loop by digital multiplication of individual modulated carriers with digital control signals. The digital control signal for each carrier may control fast variations in the transmission signal. In a TDMA system, the digital control signal may be different for each carrier and for each time slot in each frame. Static power control may be facilitated by confining the dynamic power control to the limits of the assigned static power control level or in other ways. Slow power control is done in a closed loop and includes measuring the multi-carrier signal.
According to a first embodiment of the present invention the controller preferably produces a reference signal and a power coupler in said power control loop, coupled to said analogue multi-carrier signal to produce a detected signal. The variable amplification means may be responsive to variations in the detected signal with respect to the reference signal to vary the amplification of said analogue multi-carrier signal. The variable amplification means may include a comparator, connected to a variable attenuator or amplifier in the path of the multi-carrier signal.
According to further embodiments of the present invention the power control loop preferably comprises the controller and an analogue to digital conversion means, wherein said power coupler is coupled to said analogue multi-carrier signal to produce an analogue detected signal which is converted to a digital detected signal by said analogue to digital conversion means and provided to said controller, said controller controlling the compensation of said analogue multi-carrier signal in dependence thereon.
The power control loop may effect power compensation of the analogue multi-carrier signal responsive to the combined variations of all the carriers in said analogue multi-carrier signal. In this instance, the power compensation of said analogue multi-carrier signal may be effected by varying the modulated carriers before combination to create a multi-carrier digital signal, or by varying the multi-carrier digital signal after its creation.
Alternatively, the power control loop may effect the power compensation of the analogue multi-carrier signal by compensating each of said carriers before combination responsive to the individual variations of the carriers in said analogue multi-carrier signal.
The power control means may comprise an open loop for effecting fast power variations and a closed loop for effecting slow power variations. The power control means may receive said plurality of input control signals and produce in response to each of said plurality of input control signals a power control signal for individually varying the power of each of the plurality of carriers before said combination to create said multi-carrier signal. The combination means may be a digital combination means which receives and combines digital signals to create a digital multi-carrier signal. Preferably the modulation of said first and second carriers is controlled by first and second digital signals. Preferably, each of said power control signals is a digital signal.
A plurality of second combination means may be provided, each being arranged to combine one of said modulated carriers with one of the power control signals. Digital to analogue conversion means may be arranged to convert the multi-carrier signal to an analogue signal for transmission. Each of said plurality of input control signals may be associated with a channel and its variation may be indicative of the variation of power attenuation in said channel. The power control means may be operable responsive to said input control signals to compensate for power attenuation in each channel. The power control means may further comprise a closed power control loop having detection means for detecting said multi-carrier signal to be transmitted and means responsive to the detecting means for altering the power of said multi-carrier signal responsive to said detected multi-carrier signal. Preferably, said closed power control loop compensates for slow variations or drifts in the power of said multi-carrier signal.
The detection means may detect the average power or the amplitude of the multi-carrier signal to be transmitted. The detection means may comprise a diode detector. The closed power control loop may effect power compensation of the multi-carrier signal responsive to the combined variations of all the carriers in said multi-carrier signal. The power control means may produce a reference signal, which may be analogue, and the detecting means in said closed power control loop may couple to said multi-carrier signal to produce a detected signal, said power control means being responsive to variations in the detected signal with respect to the reference signal to alter the power of said multi-carrier signal. The reference signal may be controlled by said input control signals.
The closed power control loop may comprise a comparator connected to control a variable amplifier in the path of said multi-carrier signal, said comparator receiving said detected signal and said reference signal as inputs. The closed power control loop may comprise a controller wherein said detection means couples to said multi-carrier signal to produce a detected signal which is provided to said controller which controls the altering of the power of said multi-carrier signal. The detected signal may be analogue-to-digital converted. The closed power control loop may comprise amplification means in the path of said multi-carrier signal, wherein said controller provides a compensation signal to the amplification means to compensate said multi-carrier signal. The compensation signal may be responsive to said input control signals.
The closed power control loop may comprise scaling means for amplifying said analogue multi-carrier signal and a second digital to analogue conversion means, wherein the power control loop of said control means provides a compensation signal to said scaling means via said second digital to analogue conversion means to compensate said analogue multi-carrier signal. The scaling means may be digital. The compensation signal may be provided to control the digital to analogue conversion means arranged to produce said analogue multi-carrier signal. The compensation signal may be provided to control an amplifier in the path of said analogue multi-carrier signal. The amplifier may amplify the analogue multi-carrier signal after conversion to an intermediate frequency or the variable amplifier may amplify the analogue multi-carrier signal at a radio frequency.
The power control loop may be arranged to individually compensate said plurality of carriers before their combination to create said multi-carrier signal. The control means may be arranged to individually compensate said power control signals. The controller may effect power compensation of the multi-carrier signal by compensating each of said carriers before combination responsive to the individual variations of the carriers in said multi-carrier signal. The power control means may comprise a channeliser for providing a digital detected signal in respect of each channel to said controller.
Preferably, each of said plurality of carriers has a different frequency, and, in successive predetermined periods of time, the carrier is transmitted to different receivers. The control means may vary the power of each of the plurality of carriers before combination by ramping each modulated carrier to an individually predetermined amplitude at the start of each successive predetermined period and ramps each modulated carrier downward at the end of each predetermined period. The power control means may reflect individual variation of the power of each of the carriers in dependence on the frequency of the carrier. Static power may be controlled by said closed power control loop. Static power may be controlled by said power control signals.
Embodiments of the invention are applicable to a transceiver comprising a multi-carrier radio transmitter as described hereinbefore and a receiver, wherein said input control signals are responsive to the signals received at said receiver.
The method embodying the present invention, preferably further comprises the step of: d) compensating for changes in the power level of said multi-carrier signal using a closed power control loop by: detecting said multi-carrier signal; and adjusting the power of said multi-carrier signal in dependence on said detection. In step d) the process of detecting the multi-carrier signal may comprise detecting the combined power level of the carriers in said multi-carrier signal. In step d) the process of detecting the multi-carrier signal may comprise separately detecting the power levels of each of the carriers in the multi-carrier signal.
In step d) the process of adjusting the multi-carrier signal may be effected after the combination of the plurality of carriers. In step d) the process of adjusting the multi-carrier signal may be effected by individually varying the power level of each of said plurality of carriers before their combination. The method may further comprise the step of converting the multi-carrier signal from a digital signal to an analogue signal, said adjusting of the multi-carrier being effected before said conversion step. The method may further comprise the step of converting the multi-carrier signal from a digital signal to an analogue signal, said adjusting of the multi-carrier signal being effected during or after said conversion step.
The step of detecting the power level of said multi-carrier signal may comprise coupling to the multi-carrier signal after its conversion from digital to analogue. The method may further comprise the step of upconverting said analogue multi-carrier signal to an intermediate frequency, said adjusting of the multi-carrier signal being effected after said upconversion step. The method may further comprise the step of upconverting said analogue multi-carrier signal to a radio frequency, said adjusting of the multi-carrier signal being effected after said upconversion step.
Step b) may comprise combining each one of a plurality of power control signals produced in step a) with a respective one of said plurality of modulated carrier signals. The process of adjusting the multi-carrier signal may be effected by varying each of said plurality of said power control signals. In step b) the varying of the power level of each carrier may be dependent upon the frequency of the carrier. In step c) the combining of the carriers may occur when they are digital signals.
Embodiments of the present invention find particular application in cellular radio communications networks which operate in accordance with the GSM standard.